sternocleidomastoid muscle

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A paired muscle in the superficial layers of the anterior portion of the neck, acting to flex and rotate the head.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun one of two thick muscles running from the sternum and clavicle to the mastoid and occipital bone; turns head obliquely to the opposite side; when acting together they flex the neck and extend the head